Theory Of Operation

This diagnostic is run to verify the Engine Oil Pressure (EOP) Sensor has a valid zero pressure point (offset check). The oil pressure offset reading is essentially the voltage output of the sensor at atmospheric pressure.

Two assumptions are made for this rationality:

1. The engine oil pressure will always bleed down to atmospheric pressure within a certain time period after the engine has stopped.
2. The EOP Sensor will have a stable output voltage after ignition on and before the vehicle starts to crank.

After a calibrated amount of engine off time to allow the EOP Sensor to stabilize (bleed down to atmospheric pressure) the EOP Sensor offset test will be performed at the next ignition on. A reading of the EOP Sensor output is taken when the ignition is turned to the "on" position before engine cranking. A delay time of approximately 20ms is required before the sensor is powered up and can output a valid reading.

At atmospheric pressure a "good" pressure sensor will output a voltage within an upper and lower voltage limit. The lower threshold is greater than zero volts so a circuit check can be performed on the device. The reason there is a span between the limits is because there is a small amount of air behind the diaphragm that varies in volume based on atmospheric pressure. The PCM will determine that the sensor is invalid and a failure will set if the sensor voltage is beyond normal parameters for a calibrated amount of time.